Engine: model fuel vapor behaviour

[hbeni]

From 7-28:

80°F = 26.67°C, so common in summer

Vapor lock happens when the AVGas in the fuel line is forming vapor bubbles due to boiling (30°); for example when the plane was parked in the sun at a hot day, this could happen.

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Ref:

• https://www.aopa.org/training-and-safety/students/presolo/skills/starting-a-hot-engine
• https://lipps-energie.de/_assets/img/downloads/AV-Gas.pdf AVGas boils at ~ 30°C (so 80°F remark above is a safe-sided-hint)
• Videos:
  • https://www.youtube.com/watch?v=oYV0zhX7bAk Vapor Lock
  • https://www.youtube.com/watch?v=9OI26L2SSCE Emergency in flight - Vapor Lock. Cessna 150. Successfull recovery
  • https://www.youtube.com/watch?v=cuRMt8c9yMk Rough Engine- Was It Carb Ice Or Vapour Lock?
  • https://www.youtube.com/watch?v=kfBXxWuyUnc Vapour Lock in a Cessna 152! Mayday Talk Episode 12
  • https://www.youtube.com/watch?v=zz1d4qiwgBQ https://www.youtube.com/watch?v=zz1d4qiwgBQ&t=485

[hbeni]

I think a good way to model this is to calculate:

• fuel temperature based on fuel flow and oat
  • note that the fuel will cool down a bit when traveling fast trough the narrow pipes
  • and increased fuel flow makes the pressure higher after the engine||aux-pump
• fuel boiling temp threshold based on pressure after the pumps
• Vapor forming when fuel temp < boiling temp

This model will support, "out of the box":

• Vapor forming on the ground at hot days, parked or low idle
• Vapor forming at high altitudes on hot days
• Remedy actions as described in the POH (which base on increased fuel flow, thus lowering the temps and increases pressure a bit)

Also, the aux-pump is expected to enrichen the mixture a slight bit (s. POH p. 7-28); that should be rechecked if that is already modeled because it should influence the vapor conditions (and mixture rich setting).
Enrichment from Aux-pump is implemented in #563

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An idea to model this fuel-system wise is via the (already partly simulated) mixture control unit.

• Fuel vapor is essentially a very lean mixture fed into the engine temporarily.
• Vapor forming could be modelled by a new tank containing mostly air, representing the vapor bubbles.
  • vapor forming is depending on the simulation model outlined above
  • when no vapor is forming, the tank remains empty, thus diabling the effect/consequences.
• we already have tank[6] which modelling the mixture control unit, and that was already partly used for mixture control in the priming process.
  • Mabe its easier/safer to just add another function at the end of the function chain calculating fcs/mixture-pos-norm
  • There we also could add the "aux fuel pump makes mixture slightly richer"-effect.
  • If vapor was formed, above a treshold, the vapor-air-tank "empties" promptly into the mixture tank, temproarily leaning the resulting mixture fed to the engine
• This should also result in the indicated RPM drops in Idle (note at higher RPM there is just power loss, because the prop governor will counter). We ma just utilize the "rough engine source here" too (or in addition?)